Mechanically set packers typically involve a set of drag block so that one component of two that are relatively moveable with respect to the other can be held stationary while a mandrel is moved generally axially for setting the packer. Typically a j-slot assembly connects the stationary and the movable components so that reciprocation of the string in combination with the j-slot mechanism induces a rotational movement as a pin follows a slot so that the packer is not only set but the set condition is locked. Reversal of the movement with have the pin follow the slot in reverse to release the mechanically set packer as the slips have the cones pulled out from under them that in turn allows the compressed sealing element to reduce in diameter and increase in length so that the assembly can be removed from the wellbore using the string that supports the packer mandrel.
Getting the packer to the desired location especially in a deviated borehole involves string manipulation in the axial and rotational directions where the packer body can scrape the casing or tubing. If the setting mechanism was not locked during run in it is possible that the packer manipulations to get it to the desired location could inadvertently set it. This would be disadvantageous especially if the packer was not of a retrievable design. In those cases it would require a release from the packer and another trip into the wellbore to mill it out with a mill. Even if the packer is resettable, it can be damaged by being forced to the desired location if it is in the deployed position.
The lock to prevent packer setting has to be a compact design so that it will not increase the drift dimension of the packer assembly during run in. The design also has to operate reliably and to be cost effective. One way to keep the cost down for such a lock assembly is to have it release in response to annulus pressure above a predetermined value. One such design that has been developed is shown in U.S. Pat. No. 5,320,183 where access to annulus pressure is made available upon a pressure buildup to a predetermined value in the annulus that opens a barrier to let that pressure into a chamber 30c3a that in turn pushes on an annular piston 30b3a shown in FIG. 2 so that the sleeve 30c2, 30c1 moves down and the dogs 30a move out of groove 10a. The mandrel 10 can then be manipulated. An alternative embodiment removes support for a dog extending into the mandrel to fixate it and in the process pulls a sleeve into the groove that the dog has vacated as a result of initial mandrel movement. The actuation of the sleeve to remove support for the dogs does not remove the dogs from their groove until after sleeve movement and mandrel manipulation. The subsequent covering of the groove formerly occupied by the dogs is taken to insure that the dogs cannot get back into the groove and prevent relative movement with respect to the packer that is now in the process of being set.
As packer setting locations get deeper the pressure rating of housings to withstand the higher hydrostatic forces from such depths has to be much higher than previously needed. The use of an annular piston as described above forces a reduction in wall thickness for the housing as there has to be a large annular volume to accommodate the piston and its travel distance. This forces a thinner housing wall in a location with a given drift diameter. Below certain depths such a design is not serviceable as the pressure rating on the housing cannot meet system requirements.
The present invention addresses the need for a higher housing pressure rating with a thicker wall made possible by drilling a piston bore into the housing wall in which is located a rod shaped piston. At a predetermined pressure the annulus pressure is allowed to communicate to one side of the piston that had preferably been at atmospheric pressure. The opposite side of the piston is preferably at atmospheric pressure so that the resulting piston movement liberates a c-ring that had previously extended into a mandrel groove to prevent relative movement between the housing and the mandrel. The c-ring is manufactured so that it springs away from the groove when the piston is stroked so that the c-ring will not re-engage the groove once the lock assembly is unlocked. These details and others about the present invention will be more readily apparent to those skilled in the art from a review of the details of the preferred embodiment and other embodiments described below while realizing that the full scope of the invention is to be determined by the appended claims.